1. Field of the Invention
This invention relates to polyurethane prepolymers. More particularly, the invention relates to products which can be further processed into polyurethane foams and which are particularly suitable for the formulation of moisture-hardening one-component polyurethane foams.
2. Description of the Prior Art
Polyurethane prepolymers have been known for years. In addition to numerous applications in the adhesives field, they are being used to an increasing extent in the formulation of polyurethane foams. By virtue of their insulating properties, foams such as these have grown in importance in the building industry. So-called one-component polyurethane foams are preferred for use in that field. One-component polyurethane foams are mixtures of polyurethane prepolymers, liquefying agents, accelerators, blowing agents and auxiliaries which are marketed in moisture-proof pressure vessels and which harden on contact with water or water vapor to form solid foams. Thus, according to German Application No. 20 45 852, a product such as this may be produced using a polyurethane prepolymer based on tolylene diisocyanate or crude diphenyl methane diisocyanate and a polyol having a molecular weight of at least 300.
German Application No. 27 58 114 suggests polyurethane prepolymers based on polyesters containing tertiary amino groups and/or polyether polyols containing from 2 to 8 hydroxyl groups for the same purpose.
Hitherto known polyurethane prepolymers are attended by the disadvantage that they do not dissolve conventional blowing agents, such as dichlorodifluoromethane for example, in adequate quantities. This gives rise to problems during the foaming of the mixtures, particularly in the production of lightweight foams for insulating purposes. Another disadvantage lies in the fact that the polyols on which the prepolymers are based are obtained from petroleum, i.e., they are not derived from natural, renewable raw materials. Accordingly, there is a need for polyurethane prepolymers which are based on oleochemical polyols derived from natural raw materials and which show improved solubility for fluorochloro-hydrocarbons.